Recently, in view of importance of preventing warming that is a global environmental problem, energy saving has been desired in various fields, and energy saving has also been promoted for consumer appliances. For example, for freezing refrigerators, an insulator having excellent insulation characteristics is desired in view of efficient utilization of cold heat. One example of the above-mentioned insulator is a vacuum insulator having a vacuum insulation structure.
The vacuum insulator includes a core material that retains a space, and a sheath material that insulates the space and the outside air from each other. The vacuum insulation structure is realized by eliminating the gas in the internal space of the sheath material by evacuation. As the core material, generally a powder material, a fiber material, an open-celled foam, or the like is used.
Here, recently, requirements for vacuum insulators have been diversified, and vacuum insulators with further high performance have been required.
The insulation principle of the vacuum insulator is that air which conduct heat is removed as much as possible to reduce heat conduction by gas. Accordingly, as one method for improving the insulation performance of the vacuum insulator, the internal pressure is made lower to suppress gas heat conduction due to collision of molecules. However, the vacuum degree that can be practically achieved at an industrial level using a vacuum pump is approximately 0.1 Torr (about 13.3 Pa). Therefore, it is substantially difficult to achieve a high vacuum of 0.1 Torr or more even though the internal pressure is to be made lower.
As another method for improving the insulation performance of the vacuum insulator, the gas in the vacuum insulation structure is removed. The inside of the vacuum insulator is in a state of low pressure in substantially vacuum, but some gas may be generated in a small amount from the core material, the inner surface of the sheath material, or the like, or a small amount of air may permeate/penetrate from the outside into the inside of the vacuum insulator with time. Specifically, such gas components include nitrogen, oxygen, moisture, hydrogen and the like, and existence (generation or penetration) of these gas components at the inside is a factor of causing degradation of the insulation performance of the vacuum insulator with time. Thus, when these internal gas components can be adsorbed and removed, the initial insulation performance of the vacuum insulator can be improved, and insulation performance with time can be satisfactorily maintained.
As a gas adsorbent used for this purpose, for example, a vacuum maintaining device using a Ba—Li alloy as disclosed in Patent Literature 1 has been proposed. The vacuum maintaining device has a configuration in which a first pellet formed of a Ba—Li alloy and a second pellet including a drying material are stacked and stored in a container for the purpose of maintaining vacuum in an insulating jacket, and reactivity to a gas such as nitrogen is exhibited even at room temperature.
The applicant of the present application has proposed an insulator using a gas adsorbent formed of copper ion-exchanged ZSM-5 type zeolite as disclosed in Patent Literature 2. The insulator includes at least a core material, a sheath material having a gas barrier ability, and a gas adsorbent, wherein a vacuum insulation structure is realized by decompressing the inside of the sheath material. The gas adsorbent is used for adsorbing air in an internal space of the sheath material to maintain the decompressed state of the internal space.
Copper ion-exchanged ZSM-5 type zeolite has an excellent adsorbing capability for nitrogen and oxygen as air components, or moisture. Accordingly, when the gas adsorbent is one using copper ion-exchanged ZSM-5 type zeolite, air components that cannot be eliminated by a vacuum pump, a gas generated at the inside of the vacuum insulator, and air components, moisture or the like that permeate/penetrate from the outside to the inside with time can be satisfactorily adsorbed and removed. As a result, the vacuum insulator can exhibit excellent insulation performance.
Zeolite is generally in the form of a powder, and a large number of techniques for producing a zeolite structure by molding a raw material containing zeolite have been proposed. Specifically, for example, Patent Literatures 3 and 4 disclose that zeolite particles, an inorganic binding material for binding the zeolite particles, and an organic binder are mixed, and the mixture is extrusion-molded and then fired to obtain a porous zeolite structure.